In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the tan beam yields projection data from which images in each prescribed slice may be reconstructed. Image reconstruction algorithms which may be utilized in reconstructing an image from data obtained in a helical scan are described in C. Crawford and K. King, "Computed Tomography Scanning with Simultaneous Patient Translation," Med. Phys. (716), November/December 1990, and in U.S. patent application Ser. No. 08/362,247, Helical Interpolative Algorithm For Image Reconstruction In A CT System, filed Dec. 22, 1994 and assigned to the present assignee. The known algorithms may generally be classified as Helical Extrapolative (HE) or Helical Interpolative (HI) algorithms. These algorithms typically apply a weighting function to the projection data in order to reconstruct an image. This weighting function is generally based on both the fan angle and view angle.
To achieve maximum observability of an object of interest, it is known to generate overlapping images so that one of the images centers around the object of interest. Constructing overlapping images, which typically are different views of the object of interest, is generally referred to as incremental reconstruction. Incremental reconstruction techniques are described, for example, in M. Remy-Jardin et al., "Chest evaluation with Use of Spiral Volumetric CT with the Single Breathold Technique," RSNA '91, pp. 273, November 1991, P. Costello et al., "Spiral CT of the Thorax with Small Volumes of Contrast Material: A Comparative Study" RSNA '91, pp. 274, November 1991, and D. E. Duppey, "Spiral CT of the Pancreas: Comparative Study," RSNA '91, pp. 260, November 1991.
In one known incremental reconstruction technique, an image, sometimes referred to as the "seed" image, which is common to all overlapping images is generated. When a new projection is obtained, its contribution is added to the seed image and a view corresponding to the same angular position (360.degree. apart) is subtracted from the seed image. This approach permits images to be continuously updated to generate overlapping reconstructions.
For an axial scan in which the patient remains stationary during the entire data acquisition, the amount of computation and computation time are reduced by using a seed image since a major portion of the scan data does not have to be re-processed for each view. However, with respect to a helical scan, the seed image technique is slow and cumbersome. Specifically, with helical scan data, a weighting function is applied to the projection data prior to the filtering and backprojection process. The weighting function, as explained above, is a function of both the fan angle and the view angle.
Therefore, in a helical scan context, a new image cannot be generated by simply deleting one view and adding another since the contributions from the remaining views need to be adjusted or updated, i.e., weighted. Each overlapped image therefore must be generated independently, and the amount of time required to generate overlapped images linearly increases with the number of images to be generated.
It would be desirable, of course, to reconstruct overlapping images from helical scan data without having to independently generate each overlapped image, yet substantially maintain image quality. It also is desirable to reduce the processing and total time required to reconstruct such overlapping images from helical scan data to prevent any undesirable delays.